Steven L. Smith, Bellingham, WA Home Inspector (King of the House)

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Moisture and Deck-Porch Flashings

In the wet Pacific NW, missing metal flashings at critical and weather exposed locations can result in serious consequences -- rot of siding, moisture intrusion into walls, rotted flooring.

When a flat surface such as a deck or a porch attaches to the exterior wall of the home, with few exceptions, a properly positioned metal flashing should be in place at the junction of the flat surface and the wall. Caveat, if there is a highly effective overhang roof over the landing, this may be a less critical matter but typically the flashing goes over the deck ledger board.  The photo below is a wood deck that was butted tight to the wall, a very weather exposed location, and no metal flashing has been installed.


Deck flashing

The fact that the wood deck was built directly on top of soil is not helpful either. To guard against moisture, metal flashing should be in place at the junction of a weather exposed flat surface landing and the exterior wall. Sometimes a flashing is in place, but there is an insufficient gap between the landing and the wall: Proper application of flashing includes establishing a gap, 2" minimum, between decking-floor and exterior siding or wood trim. In this photo, both the wall and the deck were rotted at various locations, so repair/replacement is necessary.

In some instances, if materials are still sound, a contractor can lift that horizontal board that's closest to the wall and, without too much trouble, install a proper flashing. The repair becomes more difficult if the decking boards run perpendicular to the wall, since installing a flashing would involve removing multiple boards.

Steven L. Smith

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Comment balloon 5 commentsSteven L. Smith • February 08 2015 02:01PM
Moisture and Deck-Porch Flashings
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In the wet Pacific NW, missing metal flashings at critical and weather exposed locations can result in serious consequences -- rot of siding, moisture intrusion into walls, rotted flooring. When a flat surface such as a deck or a porch… more